FIG. 14 is a block diagram showing a configuration of a conventional start signal generating apparatus.
The start signal generating apparatus of FIG. 14 comprises a power detecting circuit 103, a time constant circuit 104 and a threshold processing circuit 105. The power detecting circuit 103, time constant circuit 104 and threshold processing circuit 105 are turned on only when the signal level of an intermittent operation signal input to a signal input terminal 101 is H level and start their processing.
Accordingly, when the signal level of the intermittent operation signal is L level, they enter into a suspend mode, thereby being able to reduce power consumption.
FIG. 15 is a timing chart showing operation timing of the start signal generating apparatus of FIG. 14.
The operation of the start signal generating apparatus of FIG. 14 will now be described with reference to the timing chart of FIG. 15.
Although the power detecting circuit 103 is placed in a suspend mode while the signal level of the intermittent operation signal input through the signal input terminal 101 is L level, when the signal level of the intermittent operation signal becomes H level, it starts the detection processing of the wireless signal input through the signal input terminal 102.
As for the time constant circuit 104 also, although it is placed in a suspend mode just as the power detecting circuit 103 while the signal level of the intermittent operation signal input through the signal input terminal 101 is L level, it starts the processing when the signal level of the intermittent operation signal becomes H level.
More specifically, when the signal level of the intermittent operation signal becomes H level, the time constant circuit 104 imposes a frequency limit on the wireless signal detected by the power detecting circuit 103 at a preset time constant as shown in FIG. 15, and supplies the frequency-limited wireless signal to the threshold processing circuit 105.
Here, the magnitude of the time constant set by the time constant circuit 104 will determine the response speed and detection accuracy of the start signal generating apparatus.
More specifically, to increase the response speed (to achieve sharp rising characteristics), a small time constant is to be set. However, since a small time constant will broaden a limitable frequency band, it will increase a noise band and deteriorate the detection accuracy because of insufficient amount of attenuation of the harmonic components. As a result, the probability of a malfunction increases.
On the other hand, although increasing the time constant will enable improving the detection accuracy because it can limit the noise band and reduce the harmonic components sufficiently, it will reduce the response speed. In addition, since it requires to increase the time in which the signal level of the intermittent operation signal becomes H level, it will hamper reducing the power consumption of the power detecting circuit 103 and the like.
As for the threshold processing circuit 105 also, while the signal level of the intermittent operation signal input through the signal input terminal 101 is L level, although it is in the suspend mode just as the power detecting circuit 103, it starts its processing when the signal level of the intermittent operation signal becomes H level.
More specifically, when the signal level of the intermittent operation signal becomes H level, the threshold processing circuit 105 compares the level of the wireless signal on which the frequency limit is imposed by the time constant circuit 104 with a prescribed threshold as shown in FIG. 15, and decides on whether the level of the wireless signal is higher than the prescribed threshold or not.
If the threshold processing circuit 105 decides that the level of the wireless signal is higher than the prescribed threshold, it starts a post-stage communication circuit 106.
Incidentally, in addition to the foregoing start signal generating apparatus, although the following Patent Document 1 discloses a wireless receiving apparatus that reduces the power consumption by controlling the operation of a circuit in accordance with the signal level of the received signal, it does not intend to reduce the power consumption by carrying out the intermittent operation at fixed intervals. Accordingly, it must always monitor the signal level of the received signal, and hence it cannot reduce the power consumption sufficiently.